1. Field of the Invention
The present invention relates generally to a method and apparatus for forming multilayer sheets, and more particularly to a method for forming multilayer sheets for photographic printing paper supports wherein each thickness of individual layers composing the sheet is uniform widthwise and a extrusion die therefor.
2. Description of Related Art
A coextrusion apparatus is usually used for forming multilayer sheets or films for photographic printing paper supports. A feed block die cannot make each of resin layers composing a multilayer sheet have uniform thickness widthwise, in particular if there are great differences in viscosity, temperature and flow rate between melted resins, which form the resin layers. Even if a multi-manifold die, which forms relatively small unevenness of thickness of each layer, is used, it is difficult to obtain multilayer sheets with an extremely small unevenness of thickness of each layer that is desired for the multilayer sheets for the photographic printing paper supports.
To solve these problems, several techniques have been suggested for adjusting the thickness of each layer by means of a multi-manifold die. For example, Japanese Utility Model Provisional Publication No. 54-107664 discloses a multi-manifold die that adjusts the clearances in the entire passages extending from manifolds to a confluence, where the melted resins join one another through the respective passages, with use of a choke bar mechanism. Japanese Patent Provisional Publication No. 52-60853 discloses a method for manufacturing a coextrusion film. According to this method, at least one choke bar facing the confluence adjusts a confluent angle formed by the passages of the melted resins at the confluence. Japanese Utility Model Provisional Publication No. 7-15321 discloses a multilayer sheet forming apparatus providing a passage sectional area adjusting means, which forms a thin plate and pushes and pulls the thin plate along the thickness of the thin plate at the non-passage side of the thin plate, at a part in at least one of plural passages from the manifolds to the confluence. According to Japanese Patent Publication No. 60-37781, manifolds are straight, and there is provided a passage that becomes wider from the center of the die towards both ends thereof, and a slot that becomes narrower from the center of the die towards both ends thereof.
The above-mentioned techniques, however, have a problem in that it is extremely difficult to eliminate the unevenness of thickness of each of resin layers composing the multilayer sheet along the width thereof or to obtain the multilayer sheet that has a small unevenness of thickness of each layer.
In particular, the above-mentioned arts are disadvantageous to form the multilayer sheets for the photographic printing paper supports, wherein there are great differences in viscosity, temperature and flow rate between the melted resins, which form the resin layers, and it is required that the unevenness of thickness of each layer along the width thereof is .+-.1 .mu.m or less.